1. Field of the Invention
The invention relates to curable resin compositions comprising oligomers and polymers having acryloyl groups, substituted methacrylate groups and β-dicarbonyl groups. The oligomers and polymers invention are useful as binders in UV-curable and thermosetting inks and coatings.
2. Description of Related Art
Resins, containing acryloyl groups are widely used in coating industry, as for example as coating materials for paper, wood, metal and plastic, in printing inks, adhesives and sealants. The crosslinking, which includes curing or hardening, is achieved by polymerization of the acryloyl groups with electron beam or with the help of a radical initiator. Furthermore, acrylates are able to crosslink with other reactive resins, such as unsaturated polyesters, polyacetoacetates or polyamines.
The backbone of such curable systems is an acrylated oligomer or polymer, which is then later, in the cured coating or ink, responsible for hardness, toughness, solvent resistance, adhesion and so on. In the U.S. Pat. No. 5,945,489 the inventors describe such acrylated oligomers and polymers, which have acrylated groups and which are prepared by Michael addition of β-dicarbonyl compounds and excess of multifunctional acrylated monomers. The Michael addition of β-dicarbonyl compounds and acrylates requires a strong basic catalyst, having a pk of >12, such as organic amidines or inorganic bases (Organikum, V E B Deutscher Verlag der Wissenschaften, 16th edition, Berlin 1986, page 509-510). The disadvantage of this process is that the strong basic catalyst remains in the product after the production as well as in the cured coating or ink and may cause problems. It is a matter of common knowledge, that for examples strong amines may cause yellowing. Another drawback of coatings, derived from acetoacetates, acrylates and strong amines such as amidines, is their hydrolysis sensitivity. This is also described in literature (Journal of Coatings Technology, Vol. 61, No. 770, März 1989, Page 89). The authors attribute this to the high basisity of the amines, which remain in the cured product and promote ester hydrolysis in the presence of humidity. Amines having lower pk of about 10 are not suitable for the Michael addition of acrylates and β-dicarbonyl compounds. Inorganic bases such as potassium hydroxide, which can be used as well should even increase the hydrolysis sensitivity. The neutralization of the bases is often difficult, as the formed salts precipitate from the curable mixture or may “bloom out” from the cured coating.